We propose to characterize the postreplication DNA repair process in the fungus Neurospora crassa. Neurospora has been shown to lend itself well to many genetic and biochemical studies. It is the postreplication DNA repair system that is most likely involved in mutation and cancer induction. The Neurospora system may provide a useful model system for understanding this type of repair in humans. Of particular significance is the number of well characterized repair defective mutants, including excision defectives in Neurospora. Methods are being developed to extract high molecular weight DNA, radiolabeled to a high specific activity, from Neurospora directly on top of alkaline sucrose density gradients, or onto alkaline eluting filters. Changes in the molecular weight of newly synthesized DNA is known excision defective, presumed postreplication repair proficient cells, will be monitored by denaturing velocity density centrifugation or alkaline filter elution after ultraviolet irradiation. Various recovery periods will follow in the presence of or absence of caffeine. Evidence for recombination dependent verses recombination independent steps will be obtained both from the pattern of recombination defects in mutants examined, and also from an examination of the transfer of dimers from parental to nascent DNA, using the micrococcus luteus or T4 UV-light dimer specific endonuclease assay. To simplify these tasks and the analysis of data, we are developing specific DNA labeling techniques coupled with various procedures of synchronously germinated conidia and enzymatic-detergent lysing techniques. We are also attempting to adapt the procedure of alkaline elution to study this postreplication repair phenomenon. Lastly, efforts are in progress to place an osmotic mutation in the excision defective strain, in hope of producing an easy lyse strain facilitating DNA extraction, to be used in the repair studies.